The power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a semiconductor device having electrodes on a top face and a bottom face, in which electric current flows vertically. As is common in this type device, ON resistance of the power MOSFET strongly depends on electrical resistance of a drift layer, which is a conductive layer. An impurity concentration in the drift region, which determines the electrical resistance, cannot be increased beyond the limit, depending on the breakdown voltage of a pn junction between a base region and the drift layer. Thus, there is a tradeoff relationship between the breakdown voltage and the ON resistance, and there is a limit, induced by the device material, for optimizing both of the breakdown voltage and the ON resistance. Hence, it is important for making low power consumption devices to overcome this limit, and it is a way to implement a low ON resistance device beyond existing power MOSFETs.
In recent years, the MOSFET with a super junction (SJ) structure has attracted attention as the device that solves this problem, in which a p-type pillar layer and an n-type pillar layer are buried in a drift layer. Since the SJ structure includes almost the same amount of p type impurities in the p-type pillar layer with an n-type impurity in the n-type pillar layer, the drift layer becomes a pseudo non-doped layer with a balance of the negative charges in the p-type pillar and the positive charges in the n-type pillar, when being depleted. Then, it is possible to form a highly doped n-type pillar layer with low resistance, through which the current flows, while maintaining a high breakdown voltage owing to the charge balance between the n-type pillar and the p-type pillar. Thereby, a low ON resistance is implemented, which exceeds the limit of a material.
In addition, it is necessary for the power MOSFET to hold a high breakdown voltage also in a termination region provided around an active region. There is a structure that prevents a breakdown due to a variation in the amount of impurity in the termination region without forming the SJ structure.
In this structure, a low-concentration drift layer is provided in the termination region instead of the SJ structure. Further, a concentration of the impurity contained in the pillar layer in the outermost peripheral portion of the SJ structure is almost a half of the impurity concentration in the pillar layers in the portion other than the outermost peripheral portion. Hence, the power MOSFETs having such a structure are expected to have a higher breakdown voltage.